This application claims the priorities of Japanese Patent Application No. 10-21446 filed on Jan. 19, 1998 and Japanese Patent Application No. 11-2840 filed on Jan. 8, 1999, which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical system for a TV camera equipped with a still-taking function; and, in particular, to a TV camera optical system including a color-decomposing prism mounted in the main body of a broadcasting TV camera or the like.
2. Description of the Prior Art
In general, a broadcasting TV camera or the like is configured such that, for yielding a color image with a high image quality, a three-color-decomposing prism and CCDs corresponding to the respective color light components for obtaining object information carried by the decomposed individual color light components are mounted in the TV camera main body, whereas various kinds of lenses meeting the aimed purpose, such as zoom lens and macro lens, are made attachable to the front end of the TV camera main body.
In the middle of live TV broadcasting of a sports event or taking a video of a wedding ceremony, there is a demand for electrically recording a highlight scene such as a goal scene or a wedding-cake-cutting scene as a still image by the TV camera (including a video camera, ditto hereinafter) being used.
In order to fulfill such a demand, one frame of the moving picture electrically taken by the TV camera may be used as a still image. When preference is given to the compatibility with current NTSC signals, however, then thus obtained still image would exhibit a lower image quality.
Therefore, it is necessary that a TV lens section or TV camera main body be mounted with a dedicated image-capturing device for still images, such that the still images are obtained with a better image quality by the imaging system separate from the TV-image-capturing system. When two image-capturing systems are respectively provided with their lens sections, however, it becomes difficult to make the whole apparatus compact and inexpensive. Hence, it is preferred that at least a part of the TV lens section used in the TV-image-capturing system be commonly used for the still-image-capturing system.
In view of such circumstances, it is an object of the present invention to provide an optical system for a TV camera additionally equipped with a still-taking function, which can commonly use the respective optical systems of TV-image-capturing and still-image-capturing systems to the maximum.
It is also preferred that members constructed with a certain close relationship between both systems be used in optical members other than the lens sections as well, so as to make the apparatus compact and less expensive in terms of manufacturing cost.
It is thus another object of the present invention to provide an optical system for a TV camera equipped with a still-taking function, which can make the whole apparatus compact and less expensive in terms of manufacturing cost by establishing a close relationship between the respective optical systems of TV-image-capturing and still image-capturing systems.
Meanwhile, when capturing a still image, it is necessary that a part of temporally continuous object scenes be cut out with a shutter.
While an electronic shutter for changing the charge storage period of a CCD by altering the bias level or drive clock has been known, when the electronic shutter is employed, then smear may be enhanced by the light irradiated on the light-receiving surface of the CCD in the shutter-off state, whereby the image quality may deteriorate.
Though the occurrence of smear has recently been suppressed due to the development of vertical over-drain and np substrates as compared with the conventional level, further improvement is desired for those requiring a high image quality.
Therefore, it is still another object of the present invention to provide an optical system for a TV camera equipped with a still-taking function, in which there is no risk of smear being increased by light irradiating the light-receiving surface of the still-image-capturing device during the period in which the electronic shutter is closed.
Also, for example, when an electronic shutter is used in the case where the signal transmission system of the still-image-capturing device is of an interlacing type in conformity with NTSC or the like, then the charge storage periods of first and second fields constituting one frame shift from each other by {fraction (1/60)} second. As a consequence, in an object moving fast in particular, the first field picture and the second field picture may shift from each other, whereby blurring may occur when a frame image is constructed.
In view of such circumstances, it is still another object of the present invention to provide an optical system for a TV camera equipped with a still-taking function, which is free from blurring between two field images when the signal transmission system of its still-image capturing device is of an interlacing type.
When a mechanical shutter is disposed in an optical path of the still-image-capturing system in order to overcome the above-mentioned problem caused by the electronic shutter, it is necessary for the shutter member and a member for driving the shutter member to be disposed near the optical path, thus making it important to secure a space for disposing such members, which has not been necessary in the TV-image-capturing system.
Therefore, it is still another object of the present invention to provide an optical system for a TV camera equipped with a still-taking function which, when a mechanical shutter is used in the still-image-capturing system, a space for disposing its shutter member and a driving member therefor can be secured near the optical path.
The optical system for a TV camera equipped with a still-taking function in accordance with the present invention is an optical system for a TV camera having a color-decomposing prism disposed in a TV camera main body, the optical system comprising a still-image-luminous-flux-dividing prism, disposed adjacent to the color-decomposing prism, for dividing out a part of the luminous flux incident on the optical system such that thus divided part is guided to a still-image-capturing device.
Preferably, the color-decomposing prism is constituted by four blocks, one of which is used as the still-image-luminous-flux-dividing prism, whereas the remaining three blocks are used as respective color-decomposing prisms for three primary color light components.
Preferably, in the four blocks of the color-decomposing prism, the block closest to the luminous flux entrance side is used as the still-image-luminous-flux-dividing prism.
Preferably, a mechanical shutter is inserted in an optical path between the still-image-luminous-flux-dividing prism and the still-image-capturing device such as to attain an open state only during a charge storage period of the image-capturing device.
The still-image-capturing device is particularly effective when the signal transmission system is of an interlacing type.
Preferably, the luminous flux emitted from the still-image-luminous-flux-dividing prism forms an object image at a first imaging position and then, by way of a relay lens, forms the object image again on the still-image-capturing device disposed at a second imaging position, whereas a mechanical shutter is disposed between the first and second imaging positions.